Corrected link for custom operator priorities.

[RichardKelley]

In the section Custom Operators, the discussion of operator priority currently has a {tech}[priority] link to the definition of Task priority in the section Tasks and Threads. But the productions on the Custom Operators page refer to the nonterminal prio, which is defined in the section Instance Declarations. The {tech} link on the Custom Operator page should point to the location where prio is defined.

This PR changes the deftech for both instance priorities and thread priorities so that the link on the Custom Operators page points to the correct section of the manual.